1. Field of the Invention
The present invention relates to a hydraulic pressure control system for supplying brake fluid from a master cylinder to each wheel brake cylinder through a modulator by means of a fluid pump, and storing the brake fluid drained from the wheel brake cylinder through the modulator, in a reservoir.
2. Description of the Related Arts
Recent automotive vehicles are provided with a vehicle motion control system for performing various control modes such as anti-skid control, traction control, front-rear braking force distribution control, and the like. Furthermore, it is proposed to control vehicle motion characteristics by applying the braking force to each wheel irrespective of operation of a brake pedal, thereby to perform a steering control by braking, i.e., an oversteer restraining control and an understeer restraining control. With respect to a hydraulic pressure control system for use in the above-described vehicle motion control system, is often employed such a simple system that the brake fluid in a master cylinder is supplied by a fluid pump through a modulator to each wheel brake cylinder, and that the brake fluid is drained from the wheel brake cylinder through the modulator to be stored in a reservoir. For example, in order to prevent an acceleration slip from occurring in the case where an excessive driving force is applied when starting or accelerating the vehicle, a traction control system is employed, and various pressure control systems therefor are proposed in a Japanese Patent Laid-open Publication No. 64-74153. In FIG. 3 of that publication, a single changeover valve for use in the traction control system is connected to an anti-skid control circuit of so-called circulation type.
In order to perform a traction control by means of a hydraulic pressure control device for the circulation type anti-skid control, it is proposed in a Japanese Patent Laid-open Publication No. 5-116607, that a main passage is divided upstream of a section thereof connected with a pump, to provide an additional supply passage communicated with the reservoir for storing the drained fluid, and provide an intermediate reservoir communicated with the additional supply passage. And, it is so described that the intermediate reservoir is provided so as to avoid flow resistance at an input port of the master cylinder, and a resistance in a pipe or tube for connecting the master cylinder with the hydraulic pressure control device.
The hydraulic pressure control system as shown in FIG. 3 of the above Publication No. 64-74153 relates to a system in which the anti-skid control circuit is controlled when the brake pedal is not depressed. If this circuit is used for the control to be performed irrespective of the operation of the brake pedal, not only the traction control but also the steering control by braking can be performed, so that the vehicle motion control can be performed.
When the vehicle motion control is performed by the pressure control system as disclosed in the Publication No. 64-74153, it is necessary to supply sufficient brake fluid to the anti-skid control circuit irrespective of the operation of the brake pedal. However, even if a fluid pump for supplying the sufficient brake fluid was employed, it would be difficult to ensure a sufficient suction volume (or, suction volume per unit time), because the inner diameter of the pipe to be used is limited. If the inner diameter of the pipe was enlarged to increase the suction volume, its outer diameter would be necessarily enlarged, so that it would be difficult to bend it in piping. Therefore, the outer diameter of the pipe must be limited to a certain extent, to result in the limitation of the inner diameter of the pipe. As a result, the flow resistance in the pipe will be increased, so that it will be difficult to ensure the sufficient volume of the brake fluid.
According to the suction property of an ordinary pump employed in the above system as indicated by a broken line in FIG. 7, which will be described later in detail, it is impossible to introduce into the pump the fluid more than an allowable suction volume Q0, which is determined by the inner diameter of the pipe. Therefore, the problem can not be solved simply by enlarging the volume of the pump for discharging the fluid. Consequently, it will be impossible to ensure the necessary volume for discharging the fluid by a single ordinary pump.
With respect to the system disclosed in the publication No. 5-116607, the intermediate reservoir is provided for avoiding the flow resistance caused at the input port of the master cylinder. However, a delay in the normal braking operation might be caused. In the case where that system is used for the vehicle motion control when the brake pedal is not depressed, even if the brake pedal is depressed during the vehicle motion control, the pressure in the wheel brake cylinder will not be increased. In this case, therefore, the brake pedal can not be depressed further.